Method and system for monitoring EUV lithography mask flatness

ABSTRACT

Disclosed are a method of and a system for monitoring extreme ultraviolet (EUV) lithography mask flatness. An EUV mask, which is chucked to a chuck, can be scanned with a capacitance probe that generates elevation data for the EUV mask. From the elevation data, a first flatness profile can be generated. In one embodiment, the EUV mask can be rotated and rescanned.

TECHNICAL FIELD

The present invention relates generally to the field of integratedcircuit manufacture and, more particularly, to a method and a system formonitoring the flatness of an extreme ultraviolet (EUV) lithographymask.

BACKGROUND

The formation of various integrated circuit (IC) structures on a waferoften relies on lithographic processes, sometimes referred to asphotolithography, or simply lithography. For instance, patterns can beformed from a photo resist layer by passing light energy through a mask(or reticle) having an arrangement to image the desired pattern onto thephoto resist layer. As a result, the pattern is transferred to the photoresist layer. In areas where the photo resist is sufficiently exposedand after a development cycle, the photo resist material can becomesoluble such that it can be removed to selectively expose an underlyinglayer (e.g., a semiconductor layer, a metal or metal containing layer, adielectric layer, etc.). Portions of the photo resist layer not exposedto a threshold amount of light energy will not be removed and serve toprotect the underlying layer. The exposed portions of the underlyinglayer can then be etched (e.g., by using a chemical wet etch or a dryreactive ion etch (RIE)) such that the pattern formed from the photoresist layer is transferred to the underlying layer. Alternatively, thephoto resist layer can be used to block dopant implantation into theprotected portions of the underlying layer or to retard reaction of theprotected portions of the underlying layer. Thereafter, the remainingportions of the photo resist layer can be stripped.

There is a pervasive trend in the art of IC fabrication to increase thedensity with which various structures are arranged. As a result, thereis a corresponding need to increase the resolution capability oflithography systems. One promising alternative to conventional opticallithography is a next-generation lithographic technique known as extremeultraviolet (EUV) lithography where wavelengths in the range of about 11nm to about 14 nm are used to expose the photo resist layer. Forexample, using a numerical aperture of about 0.25, a wavelength of about13.4 nm and a k₁ value of about 0.6, it has been proposed that aresolution of about 32 nm can be achieved.

However, attempts to implement EUV lithography have encountered a numberof challenges. For example, mask non-flatness can result in unacceptableoverlay errors. As is known in the art, overlay relates the lateralpositioning between layers comprising an integrated circuit. If thelayers are not properly aligned with each other, the performance of thedevices of the integrated circuit can be compromised. In this situation,it is likely that the integrated circuit, if not the entire wafer (uponwhich multiple integrated circuits may be fabricated), may be unusable.

Accordingly, there exists a need in the art for techniques and systemsfor monitoring EUV lithography mask flatness.

SUMMARY OF THE INVENTION

According to one aspect of the invention, the invention is directed to amethod of monitoring flatness of an extreme ultraviolet (EUV)lithography mask. The method can include chucking the EUV mask to achuck; scanning the EUV mask with a capacitance probe to generate afirst elevation data set for the EUV mask; and generating a firstflatness profile using the first elevation data set.

According to another aspect of the invention, the invention is directedto a method of monitoring flatness of an extreme ultraviolet (EUV)lithography mask. The method can include chucking the EUV mask to achuck; scanning the EUV mask to generate a first flatness profile;removing the EUV mask from the chuck; rotating the EUV mask with respectto the chuck; rechucking the rotated EUV mask to the chuck; andrescanning the rotated EUV mask to generate a second flatness profile.

According to yet another aspect of the invention, the invention isdirected to a system for monitoring flatness of an extreme ultraviolet(EUV) lithography mask. The system can include a mask platen assemblyincluding a chuck with a mask mounting surface for receiving the EUVmask and electrostatically retaining the EUV mask to the chuck; acapacitance probe for scanning the EUV mask to generate elevation datafor the EUV mask; and a controller for receiving the elevation data andgenerating a flatness profile using the elevation data and forcontrolling the electrostatic clamping forces of the mask platenassembly.

BRIEF DESCRIPTION OF DRAWINGS

These and further features of the present invention will be apparentwith reference to the following description and drawings, wherein:

FIG. 1 is a partial schematic cross-section of one embodiment of anextreme ultraviolet (EUV) lithography mask;

FIG. 2 is a partial schematic cross-section of another embodiment of anEUV lithography mask;

FIG. 3 is a schematic block diagram of an example integrated circuitprocessing arrangement;

FIG. 4 is a schematic block diagram of an example EUV lithography maskflatness monitoring system; and

FIG. 5 is a flow diagram of an example technique for monitoring EUVlithography mask flatness.

DISCLOSURE OF INVENTION

In the detailed description that follows, similar components have beengiven the same reference numerals, regardless of whether they are shownin different views and/or embodiments. To illustrate the various aspectsof the present invention(s) in a clear and concise manner, the drawingsmay not necessarily be to scale and certain features may be shown insomewhat schematic form. Features that are described and/or illustratedwith respect to one embodiment may be used in the same way or in asimilar way in one or more other embodiments and/or in combination withor instead of the features of the other embodiments.

The description herein is presented in the exemplary context offabricating a wafer having an integrated circuit (IC) formed thereon.Example ICs include general purpose microprocessors made from thousandsor millions of transistors, a flash memory array or any other dedicatedcircuitry. However, one skilled in the art will appreciate that themethods and devices described herein can also be applied to thefabrication of any article manufactured using lithography, such asmicromachines, disk drive heads, gene chips, micro electro-mechanicalsystems (MEMS) and so forth.

The apparatus and methods described herein can provide for detection ofa key parameter for extreme ultraviolet (EUV) lithography. Namely, EUVmask flatness can be monitored to determine if conditions are favorablefor carrying out operations associated with the EUV mask, such as,illuminating a wafer with an illumination pattern defined by the EUVmask, inspecting the EUV mask (e.g., in a registration metrology tool)and/or fabricating an EUV mask using a mask writer assembly. As shouldbe appreciated, the term reticle may be used interchangeably with theterm mask.

Turning initially to FIG. 1, an EUV lithography mask 10 is illustrated.The mask can include a glass substrate 12, such as quartz glass (e.g.,SiO₂) or BPSG. A multilayer reflector film stack 14 can be formed (e.g.,by deposition) over or on an upper surface of the substrate 12. Themultilayer stack 14 can be made from alternating layers of high-Z andlow-Z materials, such as molybdenum and silicon layers (Mo/Si),molybdenum carbon and silicon layers (Mo₂C/Si), molybdenum and berylliumlayers (Mo/Be), or molybdenum ruthenium and beryllium layers (MoRu/Be).Together, the substrate 12 and multilayer stack 14 can form a maskblank.

To function as an EUV lithography mask, absorbing material can bedeposited and patterned on the multilayer stack 14 to form a pluralityof absorbers 16. Although the absorbers 16 are illustrated as individualstructures, the absorbers 16 can form an interconnected pattern. Abuffer layer (not shown) can be formed between the multilayer stack 14and the absorbing material 16 to facilitate etching of the absorbingmaterial with minimal damage to the multilayer stack 14. The absorberscan be made from chromium (Cr), titanium nitride (TiN), tantalum nitride(TaN) or other suitable material.

Alternatively, as shown in FIG. 2, a functional EUV lithography mask 10′can be formed by patterning the multilayer stack 14 of the mask blank toform a plurality of individual or interconnected multilayer reflectors14′.

The mask 10 or 10′ can include a conductive layer 18 disposed on abottom surface (or backside) of the substrate 12. The conductive layer18 can be made from a material such as chromium, silicon, or titaniumnitride. The conductive layer 18 provides a conductive plane to allowthe mask 10 or 10′ to be electrostatically clamped to a mask stageplaten (also referred to herein as a chuck).

Referring now to FIG. 3, illustrated is a schematic block diagram of anexemplary IC processing arrangement that includes an extreme ultraviolet(EUV) lithography system 20 used to image a pattern onto a wafer 22, ora region of the wafer 22. The general arrangement of the system 20 isrelatively well known in the art and will not be described in greatdetail. The system 20 can include a EUV energy source 24 for directingEUV energy 26 towards an EUV mask 28, such as the mask 10 or the mask10′. The EUV energy source 24 can include, for example, an x-ray emitter(e.g., a synchrotron or a laser plasma source) to direct x-rays into acondenser that, in turn, emits the EUV energy 26. The EUV energy canhave a wavelength of about 11 nm to about 14 nm, and in one embodiment,the wavelength can be about 13.4 nm.

The mask 28 selectively reflects EUV energy 26 such that an EUV energypattern 30 defined by the mask 28 is transferred towards the wafer 22.An imaging subsystem 32, such as a stepper assembly or a scannerassembly, sequentially directs the pattern 28 reflected by the mask 28to a series of desired locations on the wafer 22 in the form of anexposure pattern 34.

The mask 28 can be retained by an electrostatic mask platen assembly 36that includes an electrostatic chuck 38 (FIG. 4). Similarly, the wafer22 can be retained by a wafer stage platen-assembly 40. In oneembodiment, the assemblies 36, 40 can be housed in separate chambers.The assembly 36 and mask 28 can be housed in a mask chamber 42 that canbe maintained at sub-atmospheric pressure (e.g., between about 1 mTorrto about 100 mTorr). The assembly 40 and wafer 22 can be housed in awafer chamber 44 that is maintained at a pressure of below about 100mTorr. The chamber 44 can include a window (not shown) through which theexposure pattern 34 passes. The remaining elements (e.g., the imagingsubsystem 32) can be housed in one or more chambers that are kept, forexample, in vacuum to minimize attenuation of the EUV radiation.

In general, the upper surface of the mask 28 faces the upper surface ofthe wafer 22. In one embodiment, the mask 28 is inverted such that it ispulled downward by gravity, but maintained in contact with the chuck 36by clamping forces (e.g., electrostatic charges). Alternatively, themask 28 could be held in place by a vacuum chuck.

In one embodiment, the mask platen assembly 36 can manipulate theelectrostatic clamping forces between the chuck 38 and the mask 28. Forexample, non-uniform clamping force can be established from location tolocation with respect to the mask 28. Accordingly, a higher clampingforce may be applied to one point or region of the mask 28 than to asecond point or region of the mask 28.

Referring now to FIG. 4, shown is a schematic block diagram of anexample EUV lithography mask flatness monitoring system 46. Themonitoring system 46 can include a controller 48 and a probe 50. Thecontroller 48 can be, for example a general purpose or dedicatedcomputer system for executing logic instructions (e.g., in for the formof software or code) consistent with the functions described herein.Among other functions of the controller 48, the controller 48 can beprogrammed to adjust the clamping forces of the mask platen assembly 36.The probe 50 can be, for example, a non-contact capacitance gauginginstrument. Satisfactory capacitance gauges are available from ADETechnologies, 77 Rowe Street, Newton, Mass. 02466 under model numbers4810 and 5810.

The probe 50 can be disposed on or carried by a mechanical arm (notshown) such that, under the control of the controller 48, the probe 50can be moved over the mask 50 in multiple directions. By scanning theprobe 50 over the mask 28, an elevation profile, or flatness profile, ofthe mask 28 can be derived. The flatness profile can be in the form of atopographical map, flatness signature or other data compilation suchthat variations in flatness of the mask 28 can be determined along withthe relative locations of those thickness variations. For example, theprobe 50 can be maintained at a constant voltage potential relative tothe mask 28, where electrical charges can accumulate. As the distancebetween the probe 50 and the mask 28 varies, the amount of accumulatedcharge adjacent the gauge will vary, thereby producing a current thatcan be monitored. Since the monitored current is a function of thedistance between the probe 50 and the upper surface of the mask 28,changes in calculated distance (as derived from the current measurement)can be attributed to changes in flatness and recorded by the controller48.

It is noted that the mask 28 can be positioned relative to the chuck 38such that the entire mask 28 contacts the chuck 38 or only a portion(s)of the mask 28 is brought into contact with the chuck 38. In addition,the mask 28 and/or the chuck 38 need not be round.

In one embodiment, the chuck 38 can have a mask mounting surface(sometimes referred to as a reference surface) that has less than 1microrad local tilt over any 10×10 mm area. The chuck 38 can have astiffness that is much greater than a stiffness of the mask 28 such thatthe mask 28 conforms to the chuck 38.

With additional reference to FIG. 5 shown is a flow diagram of anexample technique, or process 52, for monitoring EUV lithography maskflatness. The blocks of the illustrated process 52 can be thought of asdepicting steps of a method. As should be appreciated, the blocks neednot be carried out in the order shown and some blocks may be carried outconcurrently or with partial concurrence. Also, additional blocks can beadded and certain illustrated blocks can be omitted. For example, dashedlines are used to show example alternative paths through the illustratedprocess 52 and blocks skipped by the dashed lines can be ignored.

The process 52 applies to the use of the mask 28 as illustrated in FIG.3 (e.g., the mask 28 being disposed in the integrated circuit processingassembly 20 for use in exposing the wafer 22). The process 52 alsoapplies to other situations. Examples of other situations when theprocess 52 can be carried out include during inspection of the mask 28such as in a registration metrology tool (not shown) and duringfabrication of the mask such as in a mask writer tool (not shown).

The process 52 can start in block 54 where the mask 28 is “chucked.” Forexample, the mask 28 can be brought into contact with the chuck 38 andthe mask platen assembly 36 can be controlled to electrostatically holdthe mask 28 to the chuck 38. In block 54 an initial, or default, set ofelectrostatic clamping forces can be applied to the mask 28 (e.g.,clamping force of about 15±0.15 kPa).

The process 52 can proceed to block 56 where the mask 28 is scanned withthe probe 50 to derive a flatness profile for the mask. As indicated,the flatness profile can include elevation values for various locationsacross the mask such that elevation differences in the mask 28 and therelative locations of those differences are known. The flatness profilecan be derived for the entire mask area, or in alternative embodiments,for a portion of the mask area or selected positions of the mask 28.

After the mask 28 is scanned in block 56, the process can proceed toblock 58 where the controller 48 determines if the mask 28 is flat, orhas a sufficiently flat profile within a given tolerance parameter forthe operation involving the mask 28 (e.g., illuminating the wafer 22,inspecting the mask 28, fabricating the mask 28, etc.). If the mask 28is sufficiently flat, the process 52 can end and other processesinvolving the mask 28 can be commenced (e.g., illuminating the wafer 28,inspecting registration of the mask 28, writing the mask 28, etc.).Otherwise, the process 52 can proceed to block 60.

In block 60, the mask 28 can be removed from the chuck 38. Thereafter,in block 62, the chuck 38 and/or the mask 28 can be inspected forcontamination, such as for the presence of a foreign body (e.g., aparticle). If contamination is detected on the mask mounting surface ofthe chuck 38 in block 64, the chuck 38 can be cleaned using anyappropriate technique in block 66. If contamination is detected on theupper or lower surface of the mask 28 in block 64, the mask 28 can becleaned using any appropriate technique in block 66.

If contamination is not detected in block 64 or after cleaning has beencarried out in block 66, the process 62 can proceed to block 68. Inblock 68, the mask 28 can be rotated with respect to the chuck 38. Forexample, the mask 28 can be turned about 90 degrees in either theclockwise direction or the counter-clockwise direction. However, otherangles of rotation can be used such as from about 10 degrees to about180 degrees in either the clockwise direction or the counter-clockwisedirection.

Next, in block 70, the mask 28 can be rechucked. For example, the mask28 (which is in the rotated position) can be brought into contact withthe chuck 38 and the mask platen assembly 36 can be controlled toelectrostatically hold the mask 28 to the chuck 38. In block 70 theinitial, or default, set of electrostatic clamping forces can be appliedto the mask 28. However, the clamping forces will interact with the mask28, at new corresponding locations as depending on the angle of rotationin block 68.

The process 62 can then proceed to block 72 where the mask 28 is scanneda second time with the probe 50 to derive a second flatness profile forthe mask 28. Similar to the first flatness profile, the second flatnessprofile can include elevation values for various locations across themask such that elevation differences in the mask 28 and the relativelocations of those differences are known. The second flatness profilecan be derived for the entire mask area, or in alternative embodiments,for a portion of the mask area or selected positions of the mask 28.

Proceeding to block 74, the first and the second flatness profiles canbe compared. If the variations in flatness rotated with the mask 28(e.g., higher elevations points remained in the same location relativeto the mask 28), then it can be concluded that the mask 28 is not flat.If the flatness profile rotated with the mask 28, then the process 52can proceed to block 76 where a corrective action can be taken. Anexample corrective action includes manipulating the electrostaticclamping applied to the mask 28. For example, higher elevation points onthe mask can be clamped with greater force than the initial force and/orother locations can be clamped with reduced force relative to theinitial set of forces. Another example corrective action could includerejecting the mask 28 in favor of a new mask (or a new mask blank),especially in situations where several iterations through the process 52have reached a conclusion that the mask is non-flat.

If, in block 74, the flatness profile did not rotate with the mask 28,then it can be concluded that a factor other than mask non-flatness maybe causing or contributing to the “perceived” variations in maskflatness. Other factors can include, for example, contamination, chuckirregularities, defects in the electrostatic clamping mechanism, and soforth. Therefore, if the flatness variations did not rotate with themask in block 74, the process 52 can proceed to block 78 whereappropriate action can be taken. Example action that can be taken inblock 78 include, for example, inspecting, cleaning, repairing,replacing and/or recalibrating the chuck 38 and/or mask platen assembly36. The mask 28 could also be inspected, cleaned and/or replaced.

Following blocks 76 or 78, the process 52 can return to block 56, orblock 54 if the mask 28 was removed in either of blocks 76 or 78. Aspreviously indicated, alternatives to the illustrated process 52 thatfall within the scope of the claims appended hereto can exist. At leasttwo of those alternatives are illustrated using dashed lines in FIG. 5.For example, following block 60, the process can skip checking forcontamination and/or cleaning of the chuck 38 and/or the mask 28, andproceed to block 68 where the mask 28 is rotated relative to the chuck38. As an other example, following blocks 64 and/or block 66 (e.g., evenif a positive (“Y”) determination is made in block 64), the process 52can proceed to block 70 where the mask 28 is re-chucked without rotatingthe mask 28. In this alternative, the comparison of block 74 can beperformed directly (e.g., without consideration to the angular positionof values from the second flatness profile with respect to the valuesfrom the first flatness profile).

Described above is a system and method for monitoring EUV lithographymask flatness for situations where the mask can be retained by a chuckin various devices, such as an exposure tool, a metrology tool or awriter tool. The system and method can contribute to achieving asatisfactory flatness of the EUV mask such that overlay errors presentin a wafer exposed using the EUV mask can be minimized. Mask flatness isa driving factor in minimizing overlay errors since, in EUV lithography,the target overlay error budget for in-plane and out-of-plane errors dueto mask mounting can be about 2.5 nm 3σ for exposure tools and about 2nm 3σ for writer and metrology tools. In addition, the use of a firstflatness profile scan and a second flatness profile scan can reduce theoccurrence of false positive mask flatness errors (e.g., conclusionsthat the mask is non-flat where, in fact, another issue may be causingor contributing the flatness variation reading).

Although particular embodiments of the invention have been described indetail, it is understood that the invention is not limitedcorrespondingly in scope, but includes all changes, modifications andequivalents coming within the spirit and terms of the claims appendedhereto.

1. A method of monitoring flatness of an extreme ultraviolet (EUV)lithography mask, comprising: chucking the EUV mask to a chuck; scanningthe chucked EUV mask with a contactless capacitance probe to generate afirst elevation data set for the EUV mask; generating a first flatnessprofile using the first elevation data set; comparing the first flatnessprofile against flatness tolerance parameters; and if the first flatnessprofile exceeds the flatness tolerance parameters: removing the EUV maskfrom the chuck; checking at least one of the EUV mask and the chuck forcontamination; if contamination is present, cleaning a contaminatedarea; and rechucking the EUV mask to the chuck.
 2. The method accordingto claim 1, further comprising: rescanning the EUV mask with thecapacitance probe to generate a second elevation data set for the EUVmask; and generating a second flatness profile using the secondelevation data set.
 3. The method according to claim 2, furthercomprising comparing the first flatness profile and the second flatnessprofile.
 4. The method according to claim 1, further comprising:rotating the EUV mask with respect to the chuck before rechucking theEUV mask; rescanning the rotated EUV mask with the capacitance probe togenerate a second elevation data set for the EUV mask; and generating asecond flatness profile using the second elevation data set.
 5. Themethod according to claim 4, further comprising comparing the firstflatness profile and the second flatness profile to determine ifdetected flatness variations rotated with the rotation of the EUV mask.6. The method according to claim 5, further comprising adjusting a setof electrostatic clamping forces used to retain the EUV mask to thechuck if the detected flatness variations rotated with the rotation ofthe EUV mask.
 7. The method according to claim 1, wherein the EUV maskis a reflective mask.
 8. A system for monitoring flatness of an extremeultraviolet (EUV) lithography mask, comprising: a mask platen assemblyincluding a chuck with a mask mounting surface for receiving the EUVmask and electrostatically retaining the EUV mask to the chuck; acontactless capacitance probe for scanning the EUV mask to generateelevation data for the EUV mask; and a controller for receiving theelevation data and generating a flatness profile using the elevationdata and for controlling the electrostatic clamping forces of the maskplaten assembly, wherein the controller executes logic to: conduct afirst scan of the EUV mask while chucked to generate a first flatnessprofile and, following a rotation of the EUV mask with respect to thechuck, conduct a second scan of the EUV mask while chucked to generate asecond flatness profile; and compare the first flatness profile and thesecond flatness profile to determine if detected flatness variationsrotated with the rotation of the EUV mask.
 9. The system according toclaim 8, wherein the controller executes logic to adjust a set ofelectrostatic clamping forces used to retain the EUV mask to the chuckif the detected flatness variations rotated with the rotation of the EUVmask.
 10. The system according to claim 8, wherein the EUV mask is areflective mask.